Throughout this application various publications are referenced, many in parenthesis. Full citations for these publications are provided at the end of the Detailed Description. The disclosures of these publications in their entireties are hereby incorporated by reference in this application.
The cholesterol content of the plasma is important in the development of atherosclerotic disease, both with respect to the coronary arteries and heart attacks and the cerebral arteries and strokes. Plasma cholesterol is the result of both endogenous production of cholesterol in the liver and other tissues, and exogenous absorption of cholesterol from the intestine. Intestinal cholesterol is derived both from the diet and from entero-hepatic recirculation of cholesterol.
The cellular uptake of cholesterol from the plasma utilizes a pathway which is understood in broad outline. Cholesterol and cholesterol esters are present in lipoproteins circulating in the plasma. A receptor on the cell surface recognizes protein motifs in the lipoproteins, binds the lipoprotein, and internalizes the complex in a clathrin coated pit (Havel & Kane 1995).
In contrast, cholesterol transport from the intestinal lumen is poorly understood (Wilson & Rudel 1994). Exogenous cholesterol from the diet and enteric-hepatic recirculation is important in maintaining plasma cholesterol levels (Gylling & Miettinen 1995). The intestinal lumen does not contain lipoproteins, and so the pathway for the uptake of exogenous cholesterol in the intestine must be radically different from the pathway utilized for plasma cholesterol (Tso 1985).
Early models suggested that cholesterol was incorporated into the mucosal membrane by a physical process (Tso 1985) or a cholesterol exchange. Several more recent lines of data have implicated a receptor-like protein molecule on the intestinal surface that can mediate cholesterol absorption (Thurmhofer et al. 1991; Thurmhofer & Hauser 1990). The presence of a receptor explained the specificity of the uptake, since the enterocytes transported cholesterol eight to ten fold better than the chemically very similar plant steroid, sitosterol (Ikeda et al. 1988). Cholesterol is present in the gut lumen in mixed micelles with bile salts, so that the sterol can cross the unstirred water layer in a soluble form (Thomson 1980; Westergarde & Dietschy 1976). Cholesterol esters in the diet are hydrolyzed to the free alcohol by pancreatic esterase before absorption (Gallo et al. 1977). After crossing the membrane, the cholesterol is eventually localized in a nascent chylomicron in the smooth endoplasmic reticulum (Christensen et al. 1983) where the lipids combine with apolipoprotein B to begin the assembly of the chylomicron for export into the lymphatics (Hamilton 1983).
A protein possibly involved in the absorption of cholesterol ester from rabbit intestine has been reported (Compassi et al. 1995). The protein produced by autolysis was soluble without detergents and had polypeptides of 96 and 57k daltons.
Intestinal alkaline phosphatase (Fermby 1971) is a membrane bound, glycan inositol phosphate anchored protein (Hoffman-Blume et al. 1991) of unknown physiologic function. It is bound with the anchor as well as in a form without the anchor. It is anchored to the microvillus membrane via a glycosylphosphatidylinositol linkage and is found both intracellularly and intralumenally (Brasitos et al. 1988). The enzyme is released in combination with triglycerides in the form of "surfactant-like particles" associated with the enterocyte surface and in the Golgi regions (Blume-Hoffmann et al. 1991). These particles increase during fat feeding (Mahmood et al. 1994) and are believed to be precursors of nascent chylomicrons. At alkaline pH, intestinal alkaline phosphatase can hydrolyze a number of phosphorylated substrates. The phosphatase activity of the enzyme can be inhibited by levamisole (VanBelle 1976; Metaye et al. 1988). This anthelmintic and immune response modulating drug has been used for a number of medical indications, but its mode of action is not well understood (Metaye et al. 1988; Wavwe & Janssen 1991).
A need continues to exist for an effective method to control absorption of exogenous cholesterol, in order to reduce plasma levels of cholesterol thereby reducing the risk of atherosclerotic disease.